The present invention relates to a blowing device according to the preamble of the claim 1 presented below, in a paper machine or the like, such as in a paperboard or a finishing machine or in another web treatment machine.
The invention is particularly intended to be applied in the drying sections of paper, paperboard or finishing machines or the like. The intention is then to be able to apply the invention in drying sections provided with a single wire or a twin wire run, where a wire pocket is formed between two drying cylinders and a roll below them which redirects the wire travel. An intention is also to be able to apply the invention in drying sections provided with a so called inverted run, i.e. in such drying sections where the roll turning the wire travel is arranged above the drying cylinders, or in solutions where drying cylinders are arranged above each other on two or more levels. Further the intention is to be able to apply the invention in drying sections provided with combinations of the above mentioned drying sections. The intention is further to be able to apply the invention in suitable respects in other parts of the above mentioned machines.
Previously it has been noted that the need for a negative pressure in the wire pocket is high particularly at the opening gap between the drying cylinder and the wire, so that it is possible to ensure that the wire comes off from the surface of the drying cylinder. However, an increase of the negative pressure in the whole pocket to the required negative pressure level causes certain disadvantages. Large amounts of energy must be used when the whole pocket space must be brought to the same high negative pressure level. Large air leaks may make it impossible to reach a sufficiently high negative pressure and to maintain it. So far it has generally been possible to have a sufficient increase in the negative pressure with the aid of blow boxes.
The object of the invention is typically a blowing device in a drying section which is provided with a single wire run where the web is transported between the wire and the drying cylinder over the drying cylinder. The blowing device comprises a blow box or a blow box combination extending over the whole width of the web, and the device is intended to ensure that the wire comes off from the drying cylinder in the opening nip of the wire in order to keep the web in a controlled way attached to the wire over a desired distance, even after the opening nip.
The blowing device is typically combined with means generating blowing air and arranged on that side of the wire which is away from the cylinder, mainly at the opening nip between the wire and the cylinder so that it extends, from the actual point where the wire and cylinder are disengaged, a short distance forward in the travel direction of the web. The blowing device is typically provided with two nozzles, such as gap nozzles, ejecting nozzles or the like, arranged cross-wise regarding the travel direction of the web and close to wire. The first nozzle is arranged mainly at the opening nip between the wire and the cylinder, however preferably before the actual point where the wire is disengaged from the cylinder. The second nozzle is arranged, in the travel direction of the web, at a distance from the first nozzle and the opening nip.
The nozzles are arranged in the blowing device to blow air jets away from the gap between the blowing device and the wire, so that the air jets discharged from the nozzles prevent air from entering the gap and/or suck with their ejection effect air away from the gap between the blowing device and the wire, and thus negative pressure required to support the web is maintained in the gap.
The travel of the web in the opening nip between the drying cylinder and the wire will be the more difficult to control the more the speeds of the paper machine will increase, because when the speeds increase the web will all the more tend to follow the drying cylinder. A speed increase of a few hundred meters may require a doubled negative pressure level, e.g. from a negative pressure of 500 Pa to a negative pressure of 1000 Pa.
The dry solids content of the web has also an effect on how the web comes off from the cylinder. The higher moisture the web has the more difficult it is to disengage it from the drying cylinder and the more difficult it is to achieve a good runability. A dry web will so to speak burn to the surface of the hot drying cylinder, the more easily the higher the temperature of the cylinder is. Therefore the detachment of the web from the cylinder and supporting it on the wire requires higher and higher negative pressures when the production is made more effective and the speeds are increased.
For the time being it has generally been possible to obtain a sufficiently increased negative pressure with the blow boxes. However, when the speeds increase further we get into a situation where it is cumbersome and expensive to further increase the negative pressure.
The need for the negative pressure is different in different parts of the wire pocket formed between the drying cylinders. The highest negative pressure is required at the opening nip between the cylinder and the wire for disengagement the web from the cylinder and for attaching it to the wire. In other parts of the pocket a lower negative pressure would generally be sufficient. However, with present blow box techniques we have to maintain the same negative pressure in the pockets between the drying cylinders, in the whole region over which the effect of the blow boxes extends. Large air leaks to the pocket with the negative pressure cause, particularly in fast machines, difficulties in reaching and maintaining such a particularly high negative pressure which is required at the opening nip mentioned above. Large amounts of energy must be used when the whole large pocket space must be brought to the same high negative pressure level.
Further, increasing the whole negative pressure of the pocket to a high negative pressure level may cause disadvantages. A high negative pressure may on long wire runs bend the wire, which then can come to touch surfaces of the blow box or other inflexible surfaces, and thus cause wire damages and impair the runability. A too high negative pressure in the whole pocket region can also have an effect on the web itself, and it may e.g. prevent the shrinking of the web in the cross direction too much, whereby the web may even split.
An aim is to make the travel of the web in the opening gap between the drying cylinder and the wire more secure by increasing the tension of the paper web. Tension means that a a speed difference is used to create tension in the web. However, an increased tension is not always possible, because a too high tension would decrease the tensile strength of the pare, impair the paper quality, often impair the runability and create more web breaks.
The respective negative pressure required at the paper machine in the opening nip between the drying cylinder and the web, and also in other parts of the pocket space, depends on many factors, both on production parameters and on the paper quality being produced. The requirements on the negative pressure are affected i.a. by the machine speed, the dry solid contents of the paper, the paper profile after the press, the paper quality, the paper grammage, tension differences between the press and the drying section, generally the chemistry of the wet end, the operation of the press, and the geometry and structure of the wet end. It should be possible to control the negative pressure when any of these parameters changes. It should be possible to control the negative pressure separately in the opening nip and in other regions with negative pressure.
Previously it has also been proposed to arrange a special suction box at the opening gap between the cylinder and the wire to create a higher negative pressure. The American patent publication U.S. Pat. No. 5,341,579 proposes to arrange a particular small suction box at the opening gap, with which a certain negative pressure is maintained at this point. The negative pressure at this suction box 20 and at the suction roll 12 is generated by a negative pressure blower 32. Thus they can not be controlled separately.
The American patent publication U.S. Pat. No. 5,782,009 presents a suction box mounted in the pocket between two drying cylinders, whereby the suction box is divided into two parts. The suction box part 1 having a higher negative pressure is arranged in the region of the disengaging point between the drying cylinder and the wire. The region is separated from the environment with the aid of mechanical seals. In the cross direction of the web the part 1 with the higher negative pressure can be divided into several parts, where differing negative pressures can be created in order to secure the travel of the edges of the web.
The American patent publication U.S. Pat. No. 4,359,827 presents a multi-section suction box arranged in the pocket formed between two drying cylinders. One part of the suction box is arranged in front of the wire, regarding the travelling direction of the wire at the first drying cylinder, before the disengaging point between the wire and the drying cylinder. A higher negative pressure is arranged in this section of the suction box than in the other sections of the suction box which border on the wire.
Thus, the object of the present invention is to provide an improved blowing device where the above mentioned disadvantages are minimised.
The object is particularly to provide a blowing device which makes it possible to generate a higher negative pressure at the opening nip than in other pocket regions with negative pressure.
Then the object is, for instance, to provide a blowing device with which in a drying section provided with a single wire run the negative pressure region of the pocket between the drying cylinders can be divided into two or more separately controlled regions with negative pressure.
Then the object is also to provide a blowing device with which at the opening nip the negative pressure can be controlled independently of other negative pressure control.
An object is also to provide a blowing device, to which it is possible to combine, when required, additional suction and/or blow at the opening nip.
In order to attain the above mentioned objects the invention is characterised in what is defined in the characterising clause of the first claim presented below.
A typical blowing device according to the invention comprises a blow box, in which on the side of the opening nip a throttling means, such as a blowing means or a sealing means, is arranged, in addition to the first nozzle, at a short distance from the actual disengaging point between the wire and the drying cylinder after this disengaging point. The throttling means divides the negative pressure space provided by the blowing device into two sections,
a first intensified negative pressure region bordering at the opening nip, and
a second smaller negative pressure region.
For instance in a drying section provided with a single wire run a relatively small intensified negative pressure region, at least partly isolated from the other negative pressure region, is provided with the throttling means according to the invention at the nip opening toward the pocket between the drying cylinders where the greatest need for negative pressure exists.
A typical blow box used in a drying section provided with single wire run, Uno Run Blow Box, comprises basically only a narrow box structure occupying only a part of the pocket, whereby the blow box is arranged in front of the wire run between the first drying cylinder and the turn roll. This negative pressure region is typically bordering at nozzles arranged at the top and bottom ends of the blow box, the nozzles ejecting air away from the gap-like space between the wire and the box. According to the invention a throttling means is arranged in the box between the above mentioned nozzles, so that the throttling means divides the negative pressure region generated by the box into two sections and prevents, or at least restricts, the free flow of air between these sections. The throttling means can be a simple mechanical seal which restricts the flow, or a third nozzle which is arranged to eject air away from the upper negative pressure region and to generate an intensified negative pressure region in this region. Generally it could be said that a throttling means is intended to mean all such mechanical throttling means or means provided by a nozzle that restrict the air passage between two regions being at different pressure levels. Thus the throttling means may be e.g. an ejecting nozzle, a flow restricting valve, or a curved wall projecting across the air channel which wall restricts the air flow in the channel.
If the throttling means at the border of the intensified negative pressure region is simply a mechanical, mainly non-adjustable seal, then the negative pressure in the intensified negative pressure region can be controlled for instance by adjusting the air flow of the first ejecting nozzle. The negative pressure in the intensified negative pressure region can be increased or decreased with the aid of the control. Due to the throttling means the control does not have any substantial effect in other parts of the negative pressure region.
On the other hand, if the throttling means is an ejecting nozzle the negative pressure in the intensified negative pressure region can also be controlled by controlling the air flow of this ejecting nozzle. The air which is removed by the throttling means from the intensified negative pressure region can be allowed to flow into other parts of the negative pressure region, because this amount of air is generally small compared to the size of the negative pressure region, or this removed air can be guided immediately after the nozzle completely away from the negative pressure region with the aid of guide plates or discharge channels.
Another typical blow box, Sym Run Blow Box, used in a drying section provided with a single wire run fills mainly completely the pocket defined by the input wire run, the turn roll and the output wire run, and formed between two adjacent drying cylinders. The negative pressure region is typically sealed with nozzles arranged at the front end of the blow box, i.e. mainly at the opening nip of the first drying cylinder and the wire, and at the output end of the blow box, i.e. mainly at the closing nip of the second drying cylinder and the wire. The nozzles are arranged to blow air jets outward from the negative pressure gap, so that the air jets prevent air from leaking inward to the negative pressure space. The nozzles can be so called ejecting nozzles which at the same time remove air from the negative pressure space.
According to the invention a throttling means is further arranged in the box in the region of the wire run between the first drying cylinder and the turn roll, whereby the throttling means isolates a section of the negative pressure region of the pocket into a region with an intensified negative pressure. As described above, this throttling means can be e.g. a mechanical seal or an ejecting nozzle which restricts the flow.
The separate sub-region with an intensified negative pressure according to the invention can also be provided in other negative pressure regions of the most various types, which can be generated with blowing devices. The blowing device can be a blow box, which covers a part of a wire run in a drying section provided with single or twin wire run, or which e.g. in a paper machine covers another wire run or felt run where the web is disengaged from a roll and/or is kept attached to the wire with the aid of a negative pressure, and where a smaller negative pressure region provided with an intensified negative pressure is required in addition to the conventional negative pressure.
The intensified negative pressure region is typically arranged to cover the wire run at the opening nip of a cylinder, so that the intensified negative pressure region begins at a short distance before the actual disengaging point between the cylinder and the wire, and extends a required distance forwards from the disengaging point. The greatest need for negative pressure exists particularly at the disengaging point. During the run the disengaging point may move forward or backward, so the blow box must be arranged so that the provision of a sufficient negative pressure is guaranteed during all running conditions.
In the solution according to the invention a negative pressure is maintained in the intensified negative pressure region which is typically  greater than 500 Pa, more generally xe2x89xa71,000 Pa, but however xe2x89xa620,000 Pa, preferably  less than 10,000 Pa, depending on the running situation. When required, it is of course possible to increase or decrease the negative pressure from the above mentioned values. However, the negative pressure level is typically e.g. higher than the negative pressure proll which prevails at the surface of the roll which redirects the travel of the web. In other parts of the wire pocket the negative pressure level is considerably lower, i.e. at a level of about 10 to 700 Pa, preferably 100 to 500 Pa, typically 200 to 300 Pa.
In a drying section provided with single wire run the intensified negative pressure region is a region at the opening nip with a length of about 50 to 500 mm, typically 100 to 200 mm. Then the intensified negative pressure region at cylinders nowadays in common use may start already 300 mm, often 40 to 150 mm, typically about 70 mm before the disengaging point of the wire, and it may extend about 40 to 250 mm, often 80 to 120 mm, e.g. 100 mm forwards from the disengaging point during operation. The length of the intensified negative pressure region means the distance in the travel direction of the web between two means, such as seals, throttling means, blow nozzles, which extend from the box close to the web. Between the means a higher negative pressure in the pocket space is created than in the spaces adjacent to this region.
It is, of course, possible to use a plurality of throttling means, such as e.g. mechanical seals, flow barrier plates or ejecting nozzles, to divide the negative pressure region between the box and the wire run into more than two different regions. There can be several consecutive negative pressure regions with staggered negative pressures.
The actual blowing device can comprise a single, simple box structure, or it can be formed by a plurality of structural box components. Between the structural box components there can be formed e.g. air channels in order to convey air away from a negative pressure region to another region or into the environment.
The nozzles generating the negative pressure can be simple gap nozzles which are arranged so that the air flowing out from them prevents air to penetrate into the negative pressure region and/or generates an ejecting effect at a desired point between the box and the wire. Particular ejecting nozzles can be advantageously used in the blow boxes, the nozzles being resiliently or pivotally mounted ejecting nozzles which, when required, move flexibly away from the wire, when e.g. a paper lump pushes the wire against the nozzle, so that they do not break the wire.
In order to guide air away from the intensified negative pressure region the solution according to the invention uses advantageously such surfaces which are convex and which utilising the Coanda effect can controllably direct air into a desired direction, even outside the intensified negative pressure region. With surfaces utilising the Coanda effect it is possible to direct the air, which is discharged from the intensified negative pressure region, in the lower negative pressure region toward the air discharge opening or even into the discharge opening, from which opening the air further can be discharged into a desired space by ejection or by utilising suction.
The negative pressure generated with the solution according to the invention in the intensified negative pressure region can be further intensified by arranging means creating suction in this region. The suction can be created by forming a suction opening which opens up into the blow box in this intensified negative pressure region, the suction opening communicating e.g. via a suction channel with devices creating the suction.
With the means, which are arranged in the blow box and which create suction, it is possible to control the negative pressure level in a simple way. Then the ejection blow nozzles of the box must not necessarily be controlled individually, and they can be connected to common means creating the blow.
Suction is advantageously used, particularly when the throttling means is a mechanical limiting means, which itself does not actively and in a controllable way increase the negative pressure. However, the suction can be used as an addition and to control the negative pressure also in other cases. It is advantageous to arrange a net or a corresponding device in front of the suction opening, to prevent paper lint coming into the negative pressure region from reaching the suction channels.
In contrast to the case with suction boxes, the box and wire do not come into mutual contact when suction is used in connection with the blow box solution according to the invention, where air is blown at the means defining the intensified negative pressure region between the wire and the box.